Ceramic cores are used in investment casting process to achieve complicated internal structures. They will be removed when casting are finished. The processes are: preparation cores, preparation wax pattern, preparation shells, removal the wax in shells, shell sintering, casting alloys, remove cores. With the launch of large aircraft project and the new engine development projects, more and more titanium components with complicated structure were designed to save component weight while have good integrity. Hence cores with excellent mechanical and removal properties were highly needed.
Nowadays, the widely used core materials such as alumina-based and silica-based are not suitable used in titanium investment casting due to the heavy metal/core interaction during casting process. Meanwhile, they are very difficult to take out from castings after metal solidification. Due to the excellent chemical inertness between CaO material and titanium, it attract many researchers attention. Chinese patent CN 102531648 A describes a method to prepare CaO-based ceramic core used in titanium investment casting. The core consists of 1.0˜15 wt % zirconia, 0.5˜10.0 wt % yttria, 0.05˜0.1 wt % thoria, with CaO for balance. By simply calculating, the CaO-based ceramic core in this patent contains of 74.9˜98.45 wt % CaO. Normally, the preparation time for core is about 2 weeks, and during the manufacturing process, CaO will contact or absorb the water from the environment. During this time, CaO in cores are easily absorbed or contacted with water from the environment and lead to CaO cores collapsed, therefore, it is not suitable to use in actual investment casting applications. Chinese patent CN 1793033 A claims a method to prepare the ceramic cores for titanium investment casting, which consists of 60˜85 wt % zirconia and 4˜20 wt % calcium oxide. As it was known, there're several ways to remove the cores inside the castings such as mechanical force, chemical corrosion, and the combination of both. Usually, for very complicated structures, chemical corrosion is the most effective way to remove core in castings. However, ZrO2 is hardly dissolved in acid or alkali solution, therefore, it is very difficult to remove from castings. Because of the complicated and time-consumed titanium investment casting process, cores are very easy to contact water. In order to solve CaO core water absorbance, it's necessary to develop a water-resistant CaO-based ceramic core.